README.md

Context Managers

Guarantee setup and cleanup using the with statement — even when exceptions occur.

Files

File	Description
example.py	Class-based and @contextmanager examples

---

Descriptive Example

Scenario

Temporarily enable debug mode on a config object, then automatically restore the original value.

from contextlib import contextmanager

class Config:
    debug = False

@contextmanager
def temp_value(obj, attr, new_value):
    old = getattr(obj, attr)
    setattr(obj, attr, new_value)
    try:
        yield obj
    finally:
        setattr(obj, attr, old)     # always runs, even on exception

cfg = Config()
print(cfg.debug)                    # False

with temp_value(cfg, "debug", True):
    print(cfg.debug)                # True — inside block

print(cfg.debug)                    # False — restored

Class-based version

class FileManager:
    def __enter__(self):
        self.file = open(self.path, "r")
        return self.file

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()
        return False    # False = don't suppress exceptions

---

Interview Q&A

Q1: What methods define a context manager?
A: __enter__ (setup, return value for as variable) and __exit__(exc_type, exc_val, exc_tb) (cleanup).

Q2: What happens if an exception occurs inside with?
A: __exit__ is still called. If __exit__ returns True, the exception is suppressed. If False or None, it propagates.

Q3: What does @contextmanager do?
A: Turns a generator with one yield into a context manager. Code before yield = enter; finally after = exit.

Q4: Why prefer with open(...) over manual close?
A: File is always closed, even if an exception occurs between open and close. Prevents resource leaks.

Q5: What is contextlib.ExitStack?
A: Manages a dynamic number of context managers — useful when you don't know how many resources to open at compile time.

Q6: Real-world examples of context managers?
A: File I/O (open), locks (threading.Lock), DB transactions, unittest.mock.patch, temporary directory (tempfile).

---

Run

python3 example.py